Apparatus and method for flywheel current injection for a regulator

ABSTRACT

A constant on-time regulator that may use a capacitor with low ESR without needing a series resistor is provided. A capacitor is employed to AC-couple a current sense voltage into the reference signal to provide a modified reference signal. The comparator compares the feedback voltage with the modified reference signal rather than a constant reference signal. Also, a switch may be included between the current sense voltage and the reference signal to improve load regulation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This utility patent application is a Continuation-in-Part of U.S. patentapplication Ser. No. 10/985,634, filed Nov. 10, 2004, the benefit ofwhich is hereby claimed under 35 U.S.C. §120, and is furtherincorporated herein by reference.

FIELD OF THE INVENTION

The invention is related to regulators, and in particular, to anapparatus and method for flywheel current injection for a constanton-time regulator.

BACKGROUND OF THE INVENTION

A switching regulator may be configured to provide an output voltage(Vout) in response to an input voltage (Vin). Typically, a switchingregulator includes an inductor that is coupled to a switch. Inoperation, the inductor current is a triangle wave current based on theopening and closing of the switch, and an output capacitor provides Voutfrom the inductor current. Also, the switch is controlled by a controlsignal, where the duty cycle or the frequency of the control signal istypically modulated based on negative feedback.

In a pulse width modulation (PWM) scheme, pulse width modulation istypically employed based on Vout, so that the on-time of the switch ismodulated. In a constant on-time (COT) scheme, the on-time of the switchis relatively constant, and the off-time of the switch is modulated.Unlike the PWM scheme, a COT scheme typically does not needcompensation. Also, a COT regulator typically has a relatively fasttransient response.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments of the present invention aredescribed with reference to the following drawings, in which:

FIG. 1 illustrates a block diagram of an embodiment of a regulator andexternal components;

FIG. 2 shows a block diagram of an embodiment of a regulator thatemploys diode rectification;

FIG. 3 illustrates a timing diagram of waveforms of embodiments ofsignals from FIG. 1;

FIG. 4 illustrates a timing diagram of waveforms of embodiments of themodified reference voltage, the feedback voltage, and the inductorcurrent of FIG. 1;

FIG. 5 shows a block diagram of an embodiment of the regulator of FIG. 1or FIG. 2 that includes an integrator circuit;

FIG. 6 illustrates a block diagram of an embodiment the circuit of FIG.1; and

FIG. 7 illustrates a timing diagram of waveforms of embodiments ofsignals from an embodiment of the circuit of FIG. 6, in accordance withaspects of the invention.

DETAILED DESCRIPTION

Various embodiments of the present invention will be described in detailwith reference to the drawings, where like reference numerals representlike parts and assemblies throughout the several views. Reference tovarious embodiments does not limit the scope of the invention, which islimited only by the scope of the claims attached hereto. Additionally,any examples set forth in this specification are not intended to belimiting and merely set forth some of the many possible embodiments forthe claimed invention.

Throughout the specification and claims, the following terms take atleast the meanings explicitly associated herein, unless the contextdictates otherwise. The meanings identified below do not necessarilylimit the terms, but merely provide illustrative examples for the terms.The meaning of “a,” “an,” and “the” includes plural reference, and themeaning of “in” includes “in” and “on.” The phrase “in one embodiment,”as used herein does not necessarily refer to the same embodiment,although it may. The term “coupled” means at least either a directelectrical connection between the items connected, or an indirectconnection through one or more passive or active intermediary devices.The term “circuit” means at least either a single component or amultiplicity of components, either active and/or passive, that arecoupled together to provide a desired function. The term “signal” meansat least one current, voltage, charge, temperature, data, or othersignal. Where either a field effect transistor (FET) or a bipolarjunction transistor (BJT) may be employed as an embodiment of atransistor, the scope of the words “gate”, “drain”, and “source”includes “base”, “collector”, and “emitter”, respectively, and viceversa.

Briefly stated, the invention is related to a constant on-time regulatorthat may use a capacitor with low ESR without needing a resistor inseries with output capacitor. A capacitor is employed to AC-couple acurrent sense voltage into the reference signal to provide a modifiedreference signal. The comparator compares the feedback voltage with themodified reference signal rather than a constant reference signal. Also,a switch may be included between the current sense voltage and thereference signal to improve load regulation.

FIG. 1 illustrates a block diagram of an embodiment of circuit 101,which includes regulator 100 and external components. Regulator 100includes comparator circuit 120, impedance circuit 161, capacitorcircuit C1, and switch control circuit 131. Externals components mayinclude driver circuit 191, driver circuit 192, switch circuit 111,synchronous switch circuit 112, transistor M3, inductor L1, resistors R1and R2, impedance circuit 162, output capacitor Cout, and load 150.Switch circuit 111 may include transistor M1, and synchronous switchcircuit 112 may include transistors M2.

In operation, switch circuit 111 opens and closes based on signal S1DRV.Similarly, synchronous switch circuit 112 is arranged to open and closebased on signal S2DRV. Voltage SW at switch node N9 is based on whetherswitch circuits 111 and 112 are open or closed. More specifically,switch circuit 111 couples signal VIN to node N9 if switch circuit 111is closed, and synchronous switch circuit 112 couples a ground voltageto node N9 if synchronous switch circuit 112 is closed.

Additionally, inductor L1 is arranged to provide inductor current IL tooutput capacitor Cout based, in part, on voltage SW such that outputvoltage OUT is provided. Resistors R1 and R2 are arranged to operate asa voltage divider to provide feedback voltage VFB from voltage OUT.Also, comparator circuit 120 is arranged to compare voltage VFB withmodified reference signal VREFi.

Switch control circuit 131 is arranged to provide first switch controlsignal S1CTL, and synchronous switch control circuit 132 is arranged toprovide synchronous switch control signal S2CTL. Further, driver circuit191 is arranged to provide signal S1DRV from signal S1CTL, and drivercircuit 192 is arranged to provide signal S2DRV from signal S2CTL.Transistor M3 is arranged to operate as a sense transistor such that adrain current of transistor M3 is substantially proportional to a draincurrent of transistor M2. Impedance circuit 162 is arranged to providecurrent sense voltage CS from the drain current of transistor M3. In oneembodiment, impedance circuit 162 is a resistor.

Additionally, impedance circuit 161 is arranged to provide the DCcomponent (i.e. the substantially time-independent component) of signalVREFi. Capacitor C1 is arranged to AC-couple voltage CS to node N2 toprovide the time-dependent component of signal VREFi. The effect ofvoltage CS on signal VREFi may be more easily understood in conjunctionwith the timing diagrams and accompanying explanations below. Thenegative slope of current IL while switch circuit 112 is closed may bereferred to as “flywheel current”. Capacitor C1 injects a signal that isbased on the flywheel current into node N2. Accordingly, capacitor C1may be said to perform “flywheel current injection”.

Further, comparator circuit 120 is arranged to trip if a voltageassociated with modified reference signal VREFi exceeds the feedbackvoltage VFB. In one embodiment, comparator circuit 120 provideshysteresis. In another embodiment, comparator circuit 120 does notprovide hysteresis. Comparator circuit 120 is arranged to assert signalCOMP_OUT if comparator circuit 120 is tripped.

Switch control circuit 131 is arranged to provide signals S1CTL andS2CTL based, in part, on signal COMP_OUT. In one embodiment, switchcontrol circuit 131 is arranged to provide signal S1CTL such that,whenever signal COMP_OUT is asserted, signal S1CTL is on for arelatively fixed pre-determined period of time. After the relativelyfixed pre-determined period of time elapses, signal S1CTL isde-asserted.

In one embodiment, one or more components shown as external componentsin FIG. 1 may be internal to regulator 100. For example, in oneembodiment, driver circuits 191 and 192, transistors M1-M3, andimpedance circuit 162 are included in regulator 100. In one embodiment,regulator 100 may be included on an integrated circuit, and elementsshown as external to regulator 100 may be external to regulator 100. Inanother embodiment, regulator 100 and one or more components shown asexternal to regulator 100 may be included on the integrated circuit.

As previously discussed, in one embodiment, switch control circuit 131is a constant on-time control circuit. By employing a constant on-timescheme, regulator 100 has a relatively fast response time, and does notrequire a dedicated compensation network. Also, regulator 101 may employa constant on-time scheme without requiring a resistor in series withoutput capacitor Cout in order to be stable, even if capacitor Cout hasnegligible ESR. For example, a ceramic output capacitor having acapacitance of 10 millohms or less may be employed, without the need fora resistor in series with capacitor Cout. The flywheel current injectionintroduced by employing capacitor C1 to AC-couple voltage CS reduces orsubstantially removes sub-harmonic oscillation.

Regulator 100 is illustrated as a synchronous buck regulator in FIG. 1.However, the invention is not so limited, and other topologies arewithin the scope and spirit of the invention. For example, a boostregulator, flyback regulator, or the like may be employed. Also,although an embodiment with synchronous rectification is described,diode-rectified embodiments are also included in the spirit and scope ofthe invention, as shown in FIG. 2.

FIG. 2 shows a block diagram of an embodiment of regulator 200 andexternal components. Regulator 200 is similar to regulator 100 of FIG.1, except that diode rectification is employed rather than synchronousrectification.

FIG. 3 illustrates a timing diagram of waveforms 341 and 342 ofembodiments of signal CS and signal VREFi, respectively, of FIG. 1.

When switch circuit 111 is on, switch circuit 112 is off. Accordingly,as shown by waveform 341, voltage CS is substantially zero when switchcircuit 111 is on. When switch circuit 111 turns off, inductor currentIL flows through synchronous switch circuit 112 rather than switchcircuit 111. Additionally, voltage CS is substantially proportional tocurrent IL when switch 112 is on. The constant of proportionality isnegative, so that voltage CS decreases if current IL increases.Accordingly, voltage CS falls very rapidly when switch circuit 111 turnsoff. Next, current IL ramps downward, and voltage CS ramps upwardaccordingly. The slope of the ramp is substantially given by Vo*RS/L,where Vo is the voltage associated with output signal OUT, L is theinductance associated with inductor L1, and RS is the resistanceassociated with impedance circuit 162. When switch circuit 111 turns onagain, voltage CS rapidly returns to zero, beginning the cycle again.

Signal VREFi has a DC component of substantially VREF. Additionally,signal CS is AC-coupled to node N2 to provide an AC component of signalVREFi. Accordingly, while switch circuit 111 is on, signal VREFicorresponds to approximately V1. When switch circuit 111 turns off, dueto the AC-coupling of signal CS, signal VREFi falls rapidly. Next,voltage VREFi ramps upwards with a slope that is given by substantiallyVo*RS/L. When switch circuit 111 turns on again, voltage VREFi returnsto substantially V1. Voltage V1 is based, in part, on voltage VREF.However, the voltage level that voltage VREFi returns to may changebased on a change in voltage VFB, as explained in greater detail below.

FIG. 4 illustrates a timing diagram of waveforms of embodiments of thevoltage associated with modified reference signal VREFi, feedbackvoltage VFB, and inductor current IL of FIG. 1.

Waveform 443 illustrates feedback voltage VFB at time t0, and waveform444 illustrates feedback voltage at VFB time t1. Waveform 445illustrates the voltage associated with signal VREFi at time to, andwaveform 446 illustrates the voltage associated with signal VREFi attime t1. Waveform 447 shows inductor current IL at time t0, and waveform448 shows inductor current IL at time t1.

At time t1, voltage VFB drops due to, for example, a loading increase.Because voltage VFB is lower, signal VREFi reaches voltage VFB earlierin the cycle, which reduces the off-time (Toff) of signal SCTL1. As aresult, current IL ramps to a higher level to supply the extra loading,causing voltage VFB to increase again.

Accordingly, if feedback voltage VFB decreases, the duty cycle of signalSCTL1 increases. The increase in duty cycle causes inductor current ILto increase, which in turns causes voltage VFB to increase. Thisprovides a relatively fast negative feedback loop.

The flywheel current slope modulates Toff, thus the duty cycle of signalSCTL1, while:dToff/dVFB=L/(Vo*RS), andd(duty_cycle)/dVFB=−(Freq*L)/(Vo*RS),

where duty_cycle and Freq represent the duty cycle and frequency,respectively, of signal SCTL1.

Accordingly, flywheel current injection allows for constant on-timeregulation that has both voltage-mode and current-mode characteristics.

FIG. 5 shows a block diagram of an embodiment of regulator 500.Regulator 500 may be employed as an embodiment of regulator 100 of FIG.1 or regulator 200 of FIG. 2. Regulator 500 further includes capacitorC2, resistor R3, and integrator circuit 580.

Capacitor C2 and resistor R3 are arranged to operate as a low-passfilter that provides filtered feedback voltage VFP_LP from feedbackvoltage VFB such that voltage VFP_LP substantially corresponds to the DCcomponent of voltage VFB. Additionally, integrated circuit 580 isarranged to provide corrected reference signal VREF_cor at node N3 suchthat signal VREF_cor is substantially given by:VREF_cor=VREF+K*(VREF−VFB_LP), where K is a gain factor from about twoto about four. The additional circuitry shown in FIG. 5 operates as aslow feedback loop to provide accuracy for the regulation point ofregulator 500. The additional circuitry illustrated in FIG. 5 may adjustthe DC voltage of signal VREF_cor to compensate for the effects ofloading on voltage VFB under certain operating conditions.

Accordingly, the additional circuitry illustrated in FIG. 5 may beemployed for improved regulation. The additional circuitry illustratedin FIG. 5 is not needed under most operating conditions. However, undercertain operation conditions, such as where VIN is relatively close toVOUT, load regulation may be poor without the additional circuitryillustrated in FIG. 5.

When the additional circuitry illustrated in FIG. 5 is included inregulator 500, trimming of feedback voltage VFB preferably includes theoffset voltage of integrator circuit 580 instead of comparator circuit520.

Additionally, the location of the poles contributed by integratorcircuit 580 and output capacitor COUT may be adjusted for stability.Ringing may be caused by the interaction of the poles. The ringing maybe reduced by adjusting the capacitance of output capacitor COUT suchthat the poles are farther apart.

FIG. 6 illustrates a block diagram of circuit 601, which may be employedas an embodiment of circuit 101 of FIG. 1. Regulator 600 furtherincludes transistor M4.

Transistor M4 operates as a switch that closes when switch 612 is closedand opens when switch 612 opens. Switch M4 may be included in regulator600 for improved load regulation. Switch M4 is not needed under mostoperating conditions. However, under certain operation conditions, suchas where VIN is relatively close to VOUT, load regulation may be poorwithout switch M4. Switch M4 and the additionally circuit illustrated inFIG. 5 may both be used together in one embodiment to provide betterload regulation performance than either feature alone.

FIG. 7 illustrates a timing diagram of waveforms 741 and 742,respectively, of embodiments of signals CS and VREFI from an embodimentof circuit 600 of FIG. 6. Comparing FIG. 3 with FIG. 7, it is noted thatin FIG. 7, the DC component of signal REFI is removed by switch M4,which may improve load regulation.

The above specification, examples and data provide a description of themanufacture and use of the composition of the invention. Since manyembodiments of the invention can be made without departing from thespirit and scope of the invention, the invention also resides in theclaims hereinafter appended.

1. A regulator circuit, comprising: a comparator circuit including afirst input that is coupled to a first node, a second input that iscoupled to a second node, and an output, wherein the comparator isarranged to provide a comparison output signal at the output of thecomparator based, in part, on a comparison of a feedback signal at thefirst node with a modified reference signal at the second node; animpedance circuit that is arranged to couple a reference signal to thesecond node such that the modified reference signal is based, in part,on the reference signal; a switch that is coupled between a currentsense node and another node; and a capacitor circuit that is coupledbetween said another node and the second node, wherein the capacitor isarranged to AC-couple a current sense voltage at the current sense nodeto the second node such that the modified reference signal is based, inpart, on the current sense voltage.
 2. The regulator circuit of claim 1,wherein the impedance circuit includes a resistor that is coupledbetween a reference node and the second node.
 3. The regulator circuitof claim 2, further comprising a reference voltage source that isarranged to provide a relatively constant reference voltage at thereference node.
 4. The regulator circuit of claim 2, further comprising:reference voltage source that is arranged to provide a relativelyconstant reference voltage; a low pass filter that is arranged toprovide a filtered feedback voltage from the feedback signal; and anintegrator circuit that is arranged to provide the reference signal atthe reference node based on the relatively constant reference voltageand the filtered feedback voltage such that reference signal is offsetby an offset voltage level from the relatively constant reference andsuch that the offset voltage level is substantially proportional to adifference between the relatively constant reference voltage and thefiltered feedback voltage.
 5. The regulator circuit of claim 1, furthercomprising: a switch control circuit that is coupled to the output ofthe comparator.
 6. The regulator circuit of claim 5, wherein thecomparator circuit is arranged to assert a comparison output signal if afeedback voltage associated with the feedback signal is less than amodified reference voltage associated with the modified referencesignal, and switch control circuit is arranged to assert a switchcontrol signal for a relatively fixed period of time when comparisonoutput signal is asserted, and wherein the switch is arranged to be openwhen the switch control signal is asserted.
 7. The regulator circuit ofclaim 5, further comprising: another switch that is coupled to a switchnode, wherein said another switch includes a control input; and a drivercircuit that is coupled between said another switch and the controlinput of said another switch.
 8. An apparatus, comprising: a regulatorcircuit, including: a switch including a control input, wherein theswitch is coupled between an input node and a switch node, and whereinthe control input is coupled to a switch control node; a comparatorcircuit including a first input that is coupled to a referencecomparison input node, a second input that is coupled to a feedbacknode, and an output that is coupled to a comparator output node; aswitch control circuit that is coupled to the comparator output node anda driver input node; a driver circuit that is coupled between the driverinput node and the switch control node; a second switch that is coupledbetween a flywheel current sense node and another node, wherein theregulator circuit has a flywheel current during operation of theregulator circuit, and wherein the regulator circuit is arranged suchthat a voltage at the flywheel current sense node is proportional to theflywheel current; a capacitor circuit that is coupled between saidanother node and the reference comparison node; and an impedance circuitthat is coupled between a reference signal node and the referencecomparison input node.
 9. The apparatus of claim 8, wherein thecomparator circuit is arranged to assert a comparison output signal atthe comparison output node if a feedback voltage at the feedback node isless than a modified reference voltage at the reference comparison node,and wherein the switch control circuit is arranged to assert a driverinput signal at the driver input node for a relatively fixed period oftime when comparison output signal is asserted.
 10. The apparatus ofclaim 8, further comprising a reference voltage source that is arrangedto provide a relatively constant reference voltage at the referencesignal node.
 11. A regulator circuit, comprising: a switch including acontrol input, wherein the switch is coupled between an input node and aswitch node, and wherein the control input is coupled to a switchcontrol node; a comparator circuit including a first input that iscoupled to a reference comparison input node, a second input that iscoupled to a feedback node, and an output that is coupled to acomparator output node; a switch control circuit that is coupled to thecomparator output node and a driver input node; a driver circuit that iscoupled between the driver input node and the switch control node; asecond switch that is coupled between a sense node and another node; acapacitor circuit that is coupled between said another node and thereference comparison node; an impedance circuit that is coupled betweena reference signal node and the reference comparison input node; areference voltage source that is arranged to provide a relativelyconstant reference voltage; a low pass filter that is arranged toprovide a filtered feedback voltage from a feedback signal at thefeedback node; and an integrator circuit that is arranged to provide areference signal at the reference signal node based on the relativelyconstant reference voltage and the filtered feedback voltage such thatreference signal is offset by an offset voltage level from therelatively constant reference, and such that the offset voltage level issubstantially proportional to a difference between the relativelyconstant reference voltage and the filtered feedback voltage.
 12. Amethod for regulating an output signal, comprising: providing the outputsignal by employing relatively constant on-time regulation, whereinemploying relatively constant on-time regulation includes comparing afeedback signal with a comparison signal, wherein the comparison signalis provided at a comparison node, and wherein the relatively constanton-time regulation includes opening and closing a main switch; andmodifying the comparison signal based on flywheel current injection,wherein modifying the comparison signal based on the flywheel currentinjection includes: providing a current sense signal that is based, inpart, on the flywheel current; coupling the current sense signal to acomparison node; and de-coupling the current sense signal from thecomparison node if the main switch is closed.
 13. The method of claim12, wherein modifying the comparison signal based on the flywheelcurrent injection is accomplished such that the relatively constanton-time regulation includes both current-mode characteristics andvoltage-mode characteristics.
 14. The method of claim 12, whereinproviding the output signal is accomplished, in part, with an outputcapacitor; and wherein modifying the comparison signal based on theflywheel current injection is accomplished such that sub-harmonicoscillation is substantially removed even if an equivalent seriesresistance of the output capacitor is relatively small.
 15. The methodof claim 12, wherein: providing the output signal is accomplished, inpart, with an inductor and the main switch, wherein the inductorprovides a flywheel current when the main switch is off.
 16. The methodof claim 15, wherein coupling the current sense signal to the comparisonnode is accomplished by AC-coupling the current sense signal to thecomparison node, and wherein de-coupling the current sense signal fromthe comparison node includes opening another switch between the currentsense signal and the comparison node when the main switch is off. 17.The method of claim 12, wherein providing the output signal by employingrelatively constant on-time regulation includes: opening and closing themain switch based on a switch control signal, such that an input voltageis coupled to a switch node if the main switch is closed; providing aninductor current based, in part, on a voltage at the switch node,wherein the output signal is based, in part, on the inductor current;providing the feedback signal based, in part, on the output signal;providing the comparison signal at a comparison node such that thecomparison signal is based, in part, on a reference signal; providing acomparison output signal based on comparing the comparison signal withthe feedback voltage; and asserting the switch control signal for arelatively constant period of time if the comparison output signal isasserted.
 18. The method of claim 17, wherein coupling the current sensesignal to a comparison node is accomplished by AC-coupling the currentsense voltage to the comparison node.
 19. The method of claim 18,wherein modifying the comparison signal further includes: correcting thecomparison signal to compensate for effects of loading on the feedbackvoltage.
 20. The method of claim 18, wherein providing the current sensevoltage includes: providing a current sense current that issubstantially proportional to the flywheel current; and employing aresistor to provide the current sense voltage from the current sensecurrent.